How to hide the alpha of image planes? (Images as Particles)
If you want to mix in another (non-principled) material this way, you would have to add transparency to it separately. You could do this with a Transparency material and another mix node, controlled by the image's alpha channel.
However, in your use case, I don't see why you would need another shader, especially not a glossy one - I see nothing like that in the reference image, and it wouldn't be physically accurate. Instead, I would recommend you decrease the roughness of your existing shader if you want more shine.
(Alternatively, you can use the clearcoat settings - these may work well for the natural fats that cover leaves, or to simulate wet leaves.)